A major effort in contemporary biological research is to identify genetic loci that control developmental decisions. In addition, disturbance of the normal differentiation process can contribute to tumorigenesis. Determining the key regulatory mechanisms involved in generating and maintaining the differentiated state is important for understanding both normal development and disease. This proposal is designed to investigate the control of cell type determination and differentiation in mammalian cells. By utilizing myoblast cell lines grown in vitro, and a combined somatic cell and molecular genetic approach, the regulation of the MyoD family of muscle regulatory genes will be analyzed. Intertypic hybrids will be assayed for activation and repression of myogenesis. Genetic studies of this type have been used to identify positive and negative regulatory interactions in a number of experimental systems. The experiments proposed utilize somatic cell genetics to identify genetic alterations found in rhabdomyosarcomas that inhibit muscle differentiation. In contrast to standard molecular genetic approaches, somatic cell genetics has the ability to ascertain the functional significance of complex genetic lesions, such as gene amplifications, duplications, inversions and translocations, that occur in tumor cells. By utilizing this approach, my lab has determined that alteration of the ATR locus, by isochromosome formation, inhibits differentiation in rhabdomyosarcomas. This proposal is designed to determine the molecular mechanisms by which genetic alteration of ATR results in cell-cycle abnormalities and inhibition of muscle differentiation, as well as to determine whether the i(3q)s that are present at high frequency in other types of tumor result in similar phenotypic abnormalities.